1. Field of the Invention
The present invention relates to a fiber-reinforced resin composition, and more specifically, to a resin composition having excellent resin properties for producing a composite material suitable for automobile parts, electric parts, etc., by a RIM, RRIM, SRIM or RTM method, and giving a well-molded fiber-reinforced resin composite material.
2. Related Art
Fiber-reinforced composite materials containing a plastic as a matrix are widely used in the fields of automobile parts, electric parts, construction materials, shipbuilding, sporting goods, aircraft parts, etc. As a reinforcing fiber material, glass, aramide and carbon fibers are used. As a matrix resin, a variety of matrix resins such as urethane, unsaturated polyester, vinyl ester and epoxy resins are used. And, fiber-reinforced composite materials are produced by a variety of molding or forming methods such as hand lay-up molding, filament winding, pultrusion, prepreg pressure-forming, BMC molding, SMC molding and other methods. A reaction injection molding method, which has been hitherto established, is excellent in that its molding cycle is quick and a wide variety of molded articles can be obtained by automating. In recent years, there is an increasing demand for molded articles having a large size and a complicated form, produced by this method. Molded articles produced by this method as a structural material are required to have high mechanical properties, heat resistance, durability, and the like.
Various matrix resins are conventionally used in fiber-reinforced materials produced by a reaction injection molding method. The features and defects of main matrix resins are as follows. A urethane resin is cured rapidly but is insufficient in heat resistance. An unsaturated polyester resin is also cured rapidly, but does not fully meet with other properties. A vinyl ester resin is excellent in corrosion resistance and workability, whereas, it tends to be low in composite material properties and heat resistance as compared with an epoxy resin, and causes deterioration of surface properties, etc., due to shrinkage during the curing time. Meanwhile, an epoxy resin has relatively high heat resistance and is excellent in corrosion resistance and mechanical strength. However, it has a low curing rate and a high viscosity Further, it is also pointed out that an epoxy resin is inferior in impact resistance and toughness. Although studies have been undertaken to obtain an epoxy resin having improved properties, it is difficult to satisfy all of the low viscosity and properties of heat resistance and high toughness. In the case of a fiber-reinforced resin composite material containing a reinforcing fiber, in particular, a resin cured product is required to have breaking extension which is not lower than that of the reinforcing fiber in order to fully exhibit mechanical properties inherent to the fiber itself. Especially, in order for a fiber-reinforced resin composite material to keep sufficiently high fatigue properties, a cured product is required to have considerably higher breaking strain than the fiber. For example, when a glass fiber having a breaking strain of 4% is used as a reinforcing material, a resin is required to have a breaking strain of not less than this value, and no such resin composition practically and fully meeting these conditions is known at present.
Concerning an epoxy resin, JP-A-63-218325 discloses a RIM production process using an epoxy resin containing an alicyclic amine-based curing agent and having a relatively high curing rate. Since, however, an epoxy resin having a high curing rate required for this reaction injection molding has, in general, a very high viscosity, such an epoxy resin has poor impregnability to a preliminarily placed reinforcing materials such as a continuous fiber, fabric, etc., and this poor impregnability causes voids or nonimpregnation portions in a molded article. This tendency becomes particularly outstanding when a reinforcing material is charged at a high density. And, an amine-based curing agent sometimes causes safety and health issues and deteriorates the working environment.
It is generally known to add a reactive diluent such as an aliphatic glycidyl ether, a monofunctional epoxy compound, or the like to an epoxy resin in order to decrease the viscosity of an epoxy resin for the purpose of overcoming failure in impregnation. However, the resin in which the reactive diluent has been incorporated clearly shows degradation in heat resistance, mechanical properties, water resistance, etc., and it is difficult to satisfy demanded performances. The toughness is conventionally improved by incorporating acryl rubber or an acrylonitrile-butadiene copolymer having a carboxyl group, etc., as a terminal group into an epoxy resin.
However, the above methods cause an increase in viscosity of a resin mixture and degradation in heat resistance, elastic modulus, water resistance, etc., of a cured product.
In particular, it is very difficult to obtain a resin composition having a low viscosity and a rapid curing rate which are necessary for RIM molding, satisfying heat resistance and toughness of a cured product, and having excellent durability.
JP-A-1-22848 discloses that the resin composition is improved in the viscosity and pot life and that the resultant cured product shows improved heat resistance by using a polycarboxylic acid anhydride curing agent system and a vinyl aromatic hydrocarbon. In this method, the heat resistance is improved in thermal deformation temperature, etc. However, the resultant cured product has an insufficient breaking strain, and shows degraded strength. In JP-A-63-170410, the workability and heat resistance are improved by using an imidazole compound and a radical polymerization initiator in combination. Even in this method, however, it is difficult to obtain a cured product having a sufficient breaking strain together with sufficient heat resistance.
JP-B-1-29815 and JP-B-1-29816 disclose processes for producing a prepreg or a laminated plate containing a polybasic acid anhydride curing agent system and an unsaturated polyester or epoxy vinyl ester resin. In these processes, the curing rate and the cured product properties are improved. However, the resin compositions used in these publications have a relatively high viscosity, and show poor impregnation into a reinforcing fiber when molded by a RIM method, which causes remaining voids. Further, since the resin compositions used in the above publications show a high shrinkage when molded, it is difficult to obtain a molded article having excellent surface smoothness.
Further, JP-A-1-278523 discloses a fiber-reinforced resin composite material containing an epoxy resin, glycidyl methacrylate, vinyl ester, a curing agent for the epoxy resin and a cure promoter. However, this material is insufficient for the improvement in both breaking strain or elastic modulus and heat resistance.